The invention relates to a device for coupling ionization energy into an ion or electron source that is excited inductively or inductively-capacitively.
In an ion propulsion system the plasma that is to be excited at a high frequency is located in an insulated vessel, the so-called discharge vessel. A coupling coil for feeding in a high frequency energy that is necessary for plasma excitation is wound around the discharge vessel. Thus, the plasma is located inside the coupling coil. If the impedance changes due to state changes—for example, changes in the density or the conductance—of the plasma, then the result is a mistuning of the resonant circuit.
In high frequency generators, which are driven at a fixed frequency, for example, 13.56 MHz, the mismatch, which occurs due to the plasma states-changing impedance of a coupling network, which connects the high frequency generator to the coupling coil, has to be compensated by a manual retuning of an impedance matching network (so-called matchbox) or an actuator. The result of the compensation is that the amount of the capacitance of a capacitor of the impedance matching network is suitably adjusted, for example, by changing the surface; or the inductance of a coil of the impedance matching network is changed by inserting a ferrite. Usually the impedance matching over an impedance matching network cannot be readjusted very quickly and can be optimally readjusted only over a small frequency load range. Specifically, the readjustment time of the impedance matching network can be in a range of seconds. Consequently a considerable amount of power is dissipated to some extent in the impedance matching networks.
Therefore, exemplary embodiments of the present invention provide a device for coupling ionization energy into an ion or electron source, which is excited inductively or inductively-capacitively, for use in an ion propulsion system. Furthermore, this device does not exhibit the above described drawbacks.
An inventive device for coupling ionization energy into an ion or electron source, which is excited inductively or inductively-capacitively, comprises: a discharge vessel for a gas, which is to be ionized—such as Xe, Kr, Ar, Ne, He, H2, O2, CO2, Cs or Hg; a coupling coil, which is wound around the discharge vessel and feeds in a high frequency energy, which is required for plasma excitation; a coupling capacitor, which is electrically coupled to the coupling coil; and a high frequency generator, which is electrically coupled to the coupling coil and which forms together with the at least one coupling capacitor a resonant circuit. In this case the high frequency generator exhibits a PLL (phase locked loop) controller for automatic impedance matching of the resonant circuit, so that the resonant circuit can be driven at a resonant frequency.
The coupling coil is attached to the high frequency generator and forms with the coupling capacitor of the high frequency generator a series or parallel resonant circuit.
The device of the present invention corrects the phase errors of the current and the voltage in the power output stage of the high frequency generator by automatically tracking the frequency and phase of the resonant frequency of the load circuit. The control is based on the fact that the PLL control circuit continuously compares the phase angle of the sinusoidal high frequency output current and the phase angle of the generator output voltage by way of a digital phase detector, and retunes any phase errors by resetting the generator frequency by way of a voltage-controlled oscillator (VCO) to the frequency of the resonant circuit until there is zero phase error. Since the reaction time of the PLL controller is very short (depending on the design <100 microseconds), even if the resonant frequencies change quickly, the phase errors do not persist for a prolonged period of time. Therefore, the matching of the high frequency generator to the consumer is carried out with the highest possible efficiency. Owing to the very fast frequency tracking and the phase adjustment using the digital phase comparator, the PLL controller provides that the current and the voltage are always in phase, and, thus, the maximum power can be coupled into the plasma by way of the coupling coil. This step can take place without mechanical motion or in a different way. The device of the present invention is characterized by its simplicity and high flexibility and its applicability over a wide frequency range.
Thus, the method of the present invention for optimal impedance and power matching involves adjusting the power, output by the high frequency generator, with respect to resonance and zero phase error by way of a PLL control circuit (PLL=phase locked loop) and transferring this power to the plasma. The transfer of the power with a zero phase error means that the current and the voltage in the resonant circuit are in phase; and, thus, no reactive currents flow. Therefore, there can also be no reactive power losses, as a result of which switching losses are virtually eliminated.
In order to carry out the automatic impedance matching of the resonant circuit, the current and the voltage in the resonant circuit are detected and fed to the PLL controller as the controlled variables.
The high frequency generator can operate such that resonance and optimal phase adjustment is possible. Owing to the PLL controller only sinusoidal currents flow in both the high frequency generator and in the resonant circuit and, thus, in the coupling coil. The sinusoidal current allows the high frequency generator to operate at a high efficiency, which ranges from 90 to 95%, even at high operating frequencies, that is, frequencies above 0.5 MHz.
A device with a high frequency generator with PLL control according to the present invention always works at the resonant frequency of the coupling network of the ion or electron source. The coupling network of the invention is formed by the resonant circuit that includes the coupling coil and the coupling capacitor. This means that the high frequency generator follows phase-accurately all frequency changes, independently of a frequency mistuning or the frequency bandwidth circuit quality, by means of the PLL control. The power matching of the high frequency generator occurs in the microsecond range and results, due to the exact phasing of current and voltage in the switching elements of the high frequency generator and the resonant circuit, in an almost non-dissipative switching and an optimal power coupling into the plasma.
Therefore, the inventive device is especially suited for the high frequency energy supply of ion sources (TWK) and electron sources (NTR) with inductive excitation and for applications, in which minimum energy consumption is a crucial criterion.
According to one embodiment, the PLL controller carries out a frequency and/or phase control for impedance matching of the resonant circuit. The power of the high frequency generator can be controlled by setting an input direct voltage and an input current of the high frequency generator. Therefore, the high frequency generator generates a high frequency output voltage from a direct voltage source, the voltage and current intensity of which can be controlled. This alternating voltage source is connected to a resonant circuit with the inclusion of the coupling coil, which is necessary for an inductive coupling, and the additional coupling capacitor.
In another aspect of the present invention the high frequency generator of the inventive device is connected to the coupling coil without interposing an impedance matching network, a so-called matchbox. Nevertheless, the attachment of the high frequency generator exhibiting the PLL control allows the electric energy to be coupled directly into the plasma of the ion or electron source over a wide power and frequency range.
The resonant circuit, which includes the coupling coil and the coupling capacitor, can be designed, by choice, as a series or parallel resonant circuit. In this case the impedance matching is achieved by including the coupling coil and the structural coupling capacitances between the plasma and the discharge vessel and the corresponding leads in the series or parallel resonant circuit, so that the result is an automatic frequency and phase control by means of the PLL controlled high frequency generator.
In another aspect the coupling coil can have a center tap, to which is attached the high frequency generator. This configuration allows the coupling coil to cool by feeding in a coolant without interposing insulators, because the coil ends of the coupling coil are connected to a reference potential. Water can be used as the cooling medium. The ground potential can serve, for example, as the reference potential.
In another aspect the coupling coil can be disposed between two or more coupling capacitors. In this case it is desirable for the resonant circuit, which forms, to form a resonant frequency, which is inside the lock-in frequency of the PLL controller. The high frequency generator tracks the frequency, for example, using a voltage-controlled oscillator (VCO) and a digital phase comparison between the current and the voltage in the resonant circuit until the phase error becomes zero.
Another aspect provides that the high frequency generator is connected to the coupling coil without interposing electronic components for an intermediate transformation. An alternative aspect provides that the at least one coupling capacitor and the coupling coil are attached to the high frequency generator by way of a transformer. This design may be practical especially if very extensive impedance matching is necessary. In this case the primary side the transformer is capacitively coupled to the high frequency generator and the secondary side to the at least one coupling capacitor; and the coupling coil forms the resonant circuit. It is expedient to provide a device, which detects the current and the voltage in the resonant circuit and which is coupled to the PLL controller, in order to feed to this said controller the measured current and the measured voltage as the controlled variables.
Another aspect of the invention provides that the at least one coupling capacitor is disposed in the high frequency generator or outside this high frequency generator (as an external component).
Furthermore, it can be provided that the coupling coil is grounded on one side or is operated insulated from a ground potential.
Another aspect provides that the coupling coil and the plasma form a transformer, the plasma constituting a secondary winding of the transformer.
The high frequency generator comprises a power output stage, which can be configured as one of: a half bridge class D output stage; a full bridge class D output stage; a push pull output stage; an output stage of class E; an output stage of class F; an output stage of class C. The choice as to which power output stage will be provided in the high frequency generator depends on the required frequency and power range. In all cases the impedance matching to the coupling resonant circuit is done via a frequency/phase control by the PLL controller.
Preferably class D and class E output stages are used as the output stages for the high frequency generator. These output stages are characterized by a maximum current flow angle of 180° in the switching elements of the output stages (with bipolar or MOSFET transistors). If class D output stages are used without PLL control in connection with resonant circuits, then even the smallest frequency/phase mistuning, as a function of the circuit quality of the resonant circuit, will lead to considerable resistive currents of both a capacitive or inductive nature, depending on the direction of the phase/frequency mistuning. As a result, there are very high current loads in the output stage and consequently high losses in the output stages and coupling networks. The losses occur in the form of resistive current losses, which in turn lead to a steep reduction in the power that is transmitted to the consumer. The use of the PLL control completely alleviates the aforementioned problems, that is, the phase error in the output stages, even in the case of class D, class E and class F output stages. The use of the PLL control makes it possible to totally utilize the performance of these types of output stages, that is, a current flow angle of 180°.
Owing to the high frequency generator, a resonant frequency can be set in a range of 0.5 MHz to 30 MHz. The power that is coupled into the high frequency generator is in a range of 1 W to 10 kW. The load impedance, which is coupled to the high frequency generator, is in a range of 0.1 ohm to 1 ohm or in a range of 1 ohm to 50 ohms.
In another aspect the discharge vessel of the inventive device includes a gas inlet and an outlet, which is configured opposite said gas inlet, with at least two extraction grids (each of which has one multi-apertured mask), which serves as the electric lens for focusing the ion beams to be extracted. The extraction takes place by using an electric field that is applied to the extraction grid. The discharge vessel is made of a non-conducting material exhibiting low high frequency losses, such as quartz, ceramic, Vespel or boron nitride. The discharge vessel serves as the discharge chamber for the gas that is to be ionized.
According to another aspect, the coupling coil comprises a single layered or a multi-layered or a bifilar winding. In this case the coupling coil is wound around the discharge vessel or disposed inside the discharge vessel. The coupling coil is wound about the discharge vessel in a cylindrical, conical, spherical or partially conical manner with a cylindrical transition body.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.